The molecular mechanisms that transduce light energy into a change in membrane electrical potential in rod photoreceptors remain unknown. The project proposed here attempts to understand these mechanisms by investigating the role of Ca ions as intracellular messengers that couple the excitation of rhodopsin by light to the changes in membrane potential and the role of cyclic nucleotides as regulators of the messengers. The role of Ca is to be investigated in intact retinal rods through measurements of light-dependent changes in the concentration of this ion, both in the extracellular space, with ion-specific electrodes, and in the intracellular space, with fluorescent indicator dyes. The regulator role of cyclic nucleotides is to be investigated by studying the effects of intracellular cGMP, delivered via patch electrodes, on the light-dependent changes of Ca concentration. The function of rod outer segment discs in the control of Ca concentration changes will also be studied. A new technique will be developed that should permit the analysis of the Ca uptake and release functions of discs in very nearly intact rods under conditions that minimize loss of soluble components. The possible control of the Ca transport functions of rod discs by cyclic nucleotides will also be investigated.